The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the

vestibulocochlear nerve The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the br ...

, a

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

present in

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

s, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information from the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...

of the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

directly to the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

. The other portion of the vestibulocochlear nerve is the

vestibular nerve The vestibular nerve is one of the two branches of the vestibulocochlear nerve (the cochlear nerve being the other). In humans the vestibular nerve transmits sensory information from vestibular hair cells located in the two otolith organs (the u ...

, which carries spatial orientation information to the brain from the

semicircular canals The semicircular canals are three semicircular interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the lateral, anterior and posterior semicircular canals. They are the part of the bony labyrinth, ...

, also known as semicircular ducts.

Anatomy and connections

In terms of anatomy, an auditory

nerve fiber An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see spelling differences) is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action pot ...

is either bipolar or unipolar, with its distal projection being called the peripheral process, and its proximal projection being called the

axon An axon (from Greek ἄξων ''áxōn'', axis) or nerve fiber (or nerve fibre: see American and British English spelling differences#-re, -er, spelling differences) is a long, slender cellular extensions, projection of a nerve cell, or neuron, ...

; these two projections are also known as the "peripheral axon" and the "central axon", respectively. The peripheral process is sometimes referred to as a

dendrite A dendrite (from Ancient Greek language, Greek δένδρον ''déndron'', "tree") or dendron is a branched cytoplasmic process that extends from a nerve cell that propagates the neurotransmission, electrochemical stimulation received from oth ...

, although that term is somewhat inaccurate. Unlike the typical dendrite, the peripheral process generates and conducts

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

s, which then "jump" across the cell body (or ''soma'') and continue to propagate along the central axon. In this respect, auditory nerve fibers are somewhat unusual in that action potentials pass through the soma. Both the peripheral process and the axon are

myelin Myelin Sheath ( ) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be lik ...

ated. In humans, there are on average 30,000 nerve fibers within the cochlear nerve. The number of fibers varies significantly across species; the domestic cat, for example, has an average of 50,000 fibers. The peripheral axons of auditory nerve fibers form synaptic connections with the hair cells of the

via ribbon synapses using the neurotransmitter

glutamate Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a Essential amino acid, non-essential nutrient for humans, meaning that ...

. The central axons form synaptic connections with cells in the cochlear nucleus of the brainstem. The cell bodies of the cochlear nerve lie within the cochlea and collectively form the

spiral ganglion The spiral (cochlear) ganglion is a group of neuron cell bodies in the modiolus, the conical central axis of the cochlea. These bipolar neurons innervate the hair cells of the organ of Corti. They project their axons to the ventral and dorsa ...

, named for the spiral shape it shares with the cochlea. These central axons exit the cochlea at its base and form a nerve trunk, which, in humans, is approximately one inch long. This travels in parallel with the vestibular nerves through the internal auditory canal, through which it connects to the brainstem. There, its fibers synapse with the cell bodies of the cochlear nucleus.

Types of neurons

In mammals, cochlear nerve fibers are classified as either type I or type II. * Type I neurons make up 90-95% of the neurons and innervate the inner hair cells. They have relatively large diameters, are bipolar, and are myelinated. Each type I axon innervates only a single inner hair cell, but each inner hair cell is innervated by up to 30 such nerve fibers, depending on species and location within the cochlea. * Type II neurons make up the remaining 5-10% of the neurons and innervate the outer hair cells. They have relatively small diameters, are unipolar, and are unmyelinated.

Cochlear nuclear complex

In mammals, the axons from each cochlear nerve terminate in the cochlear nuclear complex that is ipsilaterally located in the medulla of the brainstem. The cochlear nucleus is the first 'relay station' of the central auditory system and receives mainly ipsilateral afferent input. The three major components of the cochlear nuclear complex are (see figure below): * the dorsal cochlear nucleus (DCN) * the anteroventral cochlear nucleus (AVCN) * the posteroventral cochlear nucleus (PVCN) Each of the three cochlear nuclei are organized to sort sound according to a specific spacial arrangement. As such, sound frequencies detected by the cochlea are transmitted electrically to specific positions in the cochlear nuclei. The axons from the low-frequency region of the cochlea project to the ventral portion of the dorsal cochlear nucleus and the ventrolateral portions of the anteroventral cochlear nucleus. The axons from the high-frequency region project to the dorsal portion of the anteroventral cochlear nucleus and the uppermost dorsal portions of the dorsal cochlear nucleus. The axons from the intermediate frequency region project to intermediate targets. Through this process, a spatial representation of sound is created by electrical nerve impulses through the cochlear complex.

References

Additional images

File:Ear-anatomy-text-small-en.svg, Ear anatomy File:Cochlear_nucleus_innervated_by_a_branching_auditory_nerve_fibre.JPG, Cochlear nucleus innervated by a branching auditory nerve fibre File:Gray761.png, Terminal nuclei of the vestibular nerve, with their upper connections. File:Gray903.png, Transverse section of the cochlear duct of a fetal cat. File:Gray929.png, Floor of ductus cochlearis. file:Gray928.png, Diagrammatic longitudinal section of the cochlea file:Organ of corti.svg,

Organ of Corti The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Trans ...

External links